There are known bending machines such as press-brakes in which a sheet-like workpiece is bent in such a way that information, such as the thickness and material of the workpiece and a target bending angle, is input in an NC device; the amount of lowering an upper bender (punch) is calculated based on the input information; and lowering of the upper bender is controlled according to the calculated value, whereby a desired product is produced.
In spite of controlling the amount of lowering the upper bender by the use of the NC device as described above, the bending machines often fail in bending a workpiece at a desired angle, because of variations in the characteristics of materials themselves such as thickness, Young's modulus and n-value as well as various bending conditions. One attempt that has been made to solve the above problem is that trial is carried out by manual control of the upper bender prior to real bending in order to determine a control amount for lowering the upper bender and the control amount thus determined is input in the NC device. However, such a fine adjustment of the amount of lowering the upper bender has to be carried out each time a material lot is changed, and it is therefore very troublesome.
In order to achieve high-accuracy bending by eliminating bending angle errors caused by the above-mentioned variations in the characteristics of materials etc., the following solutions have been proposed.
(i) The characteristics of a material are obtained from "load-displacement data" which can be obtained in the course of bending. From these characteristics, an angle at which the material is bent is predicted. PA1 (ii) An angle at which a workpiece is bent is directly detected in the course of bending. PA1 (a) projector means for projecting spot lights onto an object to be photographed thereby forming bright points on the surface thereof; PA1 (b) photographing means for photographing the object to be photographed on the surface of which bright points are formed by the projector means; PA1 (c) coordinate transformation parameter calculating means for forming a virtual cubic lattice in space by moving, at a specified pitch, a lattice pattern which is formed by arranging a number of reference positions in the pattern of a lattice on the surface of the object to be photographed, and for calculating a coordinate transformation parameter from a plurality of images obtained by photographing each point of the cubic lattice, the parameter being used for transformation between a space coordinate system in which each axis of the cubic lattice is set as a reference axis and a plane coordinate system which represents an image plane of the photographing means; PA1 (d) linear equation calculating means for calculating a linear equation representing, in the space coordinate system, each spot light projected by the projector means from the positions of at least two bright points formed in the lattice patterns of the cubic lattice; PA1 (e) coordinate value transforming means for photographing the bent workpiece on the outer faces of which at least three bright points are formed by projecting at least three spot lights thereon by means of the projector means and for transforming the coordinate values of each photographed bright point in the plane coordinate system to those of the space coordinate system by the use of the coordinate transformation parameter calculating means and the linear equation calculating means; and PA1 (f) bending angle calculating means for calculating a planar equation representing the three bright points from the coordinate values of the three bright points which have been transformed to the space coordinate system by the coordinate value transforming means, thereby obtaining an angle at which the workpiece is bent. PA1 (a) projector means for projecting slit lights onto an object to be photographed thereby forming bright lines on the surface thereof; PA1 (b) photographing means for photographing the object to be photographed on the surface of which bright lines are formed by the projector means; PA1 (c) coordinate transformation parameter calculating means for forming a virtual cubic lattice in space by moving, at a specified pitch, a lattice pattern which is formed by arranging a number of reference positions in the pattern of a lattice on the surface of the object to be photographed, and for calculating a coordinate transformation parameter from a plurality of images obtained by photographing each point of the cubic lattice, the parameter being used for transformation between a space coordinate system in which each axis of the cubic lattice is set as a reference axis and a plane coordinate system which represents an image plane of the photographing means; PA1 (d) planar equation calculating means for calculating a planar equation representing, in the space coordinate system, each slit light projected by the projector means from the positions of at least two bright lines formed in the lattice patterns of the cubic lattice; PA1 (e) coordinate value transforming means for photographing the bent workpiece on the outer faces of which at least two bright lines are formed by projecting at least two slit lights thereon by means of the projector means and for transforming a linear equation representing each photographed bright line in the plane coordinate system to that of the space coordinate system by the use of the coordinate transformation parameter calculating means and the planar equation calculating means; and PA1 (f) bending angle calculating means for calculating a planar equation representing the two bright lines from the linear equations for the two bright lines which have been transformed to the space coordinate system by the coordinate value transforming means, thereby obtaining an angle at which the workpiece is bent.
The first method, in which a bending angle is predicted from "load-displacement data" obtained during bending, includes approximation in the process of calculation, because most of the calculation is performed based on the theory of simple bending. Therefore, a satisfactorily accurate result cannot be expected and in fact, the above method has not come into practice yet.
In contrast with the first one, the second method in which the bending angle of a workpiece is directly detected in the course of bending is easy to carry out since a workpiece, i.e., the object to be controlled itself is directly measured and therefore has high feasibility.
Conventionally, there are two types of detecting mechanisms for detecting an angle at which a workpiece is bent. One is the contact-type and the other is the non-contact type.
As one example of the contact-type detecting mechanism, a continuous follow-up angle detector is disclosed in Japanese Patent. Publication Laid Open No. 273618 (1989). In this detector, a rectangular link is utilized and the inclination of a probe being in contact with the inclined surface of a metal sheet (workpiece) is read by an encoder provided in the link mechanism, thereby detecting an angle at which the metal sheet is bent.
As the non-contact detecting mechanism, there is generally known a method in which a plurality of distance sensors are employed for measuring the distance from each sensor to the bent part of a workpiece and the difference between the distances measured is obtained, whereby the bending angle of a workpiece is detected. One example of such a detecting mechanism is disclosed in Japanese Patent Publication Laid Open No. 49327 (1988), in which overcurrent sensors are employed as the distance sensors. Another example is disclosed in Japanese Patent Publication Laid Open No. 2723 (1989) in which electrostatic capacity sensors are employed as the distance sensors. Also, Japanese Patent Publication Laid Open No. 271013 (1989) and West German Patent No. 3216053 disclose the non-contact type detectors in which optical sensors are used as the distance sensors.
However, the above conventional bending angle detecting mechanisms present the following problems.
Firstly, the contact-type detecting mechanisms cannot be suitably used when bending a workpiece with short legs, as they require comparatively long legs to ensure a high measuring accuracy. Further, if the contact-type mechanisms are used for long time, long contact with workpieces causes the probe to be worn and deformed, resulting in a decreased measuring accuracy.
In the non-contact type detecting mechanisms, a plurality of distance sensors are employed for measuring and calculating the distance from each sensor to a bent workpiece, but a long distance cannot be kept between one sensor to another so that a satisfactory detecting accuracy cannot be obtained. Further, the non-contact type mechanisms including overcurrent sensors or electrostatic capacity sensors have the disadvantage that since the outputs from the sensors vary depending on the material of a workpiece, measuring conditions have to be changed each time a different material is used. The non-contact type mechanisms including optical sensors also have the disadvantage that light directed to the surface of a workpiece disperses in some surface conditions, which leads to increased measuring errors and a decreased measuring accuracy. Another disadvantage of the above type is that the measuring accuracy is dependent on sensors to be used and the resolving power of the image receptor.